This invention relates to a hydraulic drive mechanism to actuate a tool for joining at least a pair of workpieces of ductile material, in particular for self-piercing riveting or clinching.
Drive mechanisms such as disclosed in EP 0 675 774 B1 generally comprise a punch drive for actuating a punch to perform the joining operation and a clamp drive for actuating a clamp or nose clamping the workpieces against an up-setting die during the joining operation. Both drives comprise piston cylinder assemblies. The cylinders of both drives are fixedly connected to each other, and the piston of the clamp drive is concentrically arranged between the piston rod of the punch drive and the cylinder of the clamp drive such that the piston of the clamp drive is telescopically displaceable by being pressurized via a pressure chamber with a fluid connection in order to generate the clamping force. While a spring is used to reset the piston of the clamp drive, the drive mechanism still requires three fluid connections (two for the punch drive and one for the clamp drive) resulting in a somewhat complex structure of the drive mechanism and its hydraulic supply system.
Furthermore it has become known to use a spring supported against the piston of the punch drive to generate the clamping force. While this simplifies the structure of the drive mechanism and in particular of its hydraulic supply system, this drive mechanism does not allow to generate a uniform clamping force because the spring force varies in response to movements of the punch.
It is a primary object of the present invention to provide a hydraulic drive mechanism to actuate a tool for joining at least two workpieces of ductile material, which drive mechanism is of simple structure and yet allows to generate a uniform hydraulic clamping force for clamping the workpieces against an up-setting die.
In the hydraulic drive mechanism of the present invention the clamp drive is carried by the piston rod of the punch drive and the pressure chamber of the clamp drive communicates with a second work chamber of the punch drive via a flow passage such that pressure from the second work chamber of the punch drive is transmitted to the pressure chamber of the clamp drive in order to generate the clamping force.
Preferably the fluid connection of the second work chamber of the punch drive is arranged to communicate with a low pressure region via a pressure limiting valve in order to limit, via said flow passage, the pressure within the pressure chamber of the clamp drive to thereby limit the clamping force.
The clamping force is generated by the pressure in one of the work chambers of the punch drive. As a result thereof only two fluid connections are required to operate the drive mechanism. A further advantage of the invention is that it is only the punch drive which has to be lengthened in order to increase the feed stroke of the clamp and the punch, because a respective lengthening of the feed stroke of the clamp drive carried by the punch drive will automatically result therefrom. This will result altogether in easier handling of and improved access to the drive mechanism. Furthermore, the drive mechanism of the present invention is of low manufacturing and assembly costs resulting from the simplified structure, the reduced number of stroke dependent parts and the simplified hydraulic supply system.